Commit Message

We call a C helper to save all svcpu fields into our vcpu. The C
ABI states that r12 is considered volatile. However, we keep our
exit handler id in r12 currently.
So we need to save it away into a non-volatile register instead
that definitely does get preserved across the C call.
This bug usually didn't hit anyone yet since gcc is smart enough
to generate code that doesn't even need r12 which means it stayed
identical throughout the call by sheer luck. But we can't rely on
that.
Signed-off-by: Alexander Graf <agraf@suse.de>
---
arch/powerpc/kvm/book3s_interrupts.S | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)

On 30.11.2013, at 08:20, Paul Mackerras <paulus@samba.org> wrote:
> On Fri, Nov 29, 2013 at 03:54:58AM +0100, Alexander Graf wrote:>> We call a C helper to save all svcpu fields into our vcpu. The C>> ABI states that r12 is considered volatile. However, we keep our>> exit handler id in r12 currently.>> >> So we need to save it away into a non-volatile register instead>> that definitely does get preserved across the C call.>> >> This bug usually didn't hit anyone yet since gcc is smart enough>> to generate code that doesn't even need r12 which means it stayed>> identical throughout the call by sheer luck. But we can't rely on>> that.>> >> Signed-off-by: Alexander Graf <agraf@suse.de>>> --->> arch/powerpc/kvm/book3s_interrupts.S | 15 +++++++++++---->> 1 file changed, 11 insertions(+), 4 deletions(-)>> >> diff --git a/arch/powerpc/kvm/book3s_interrupts.S b/arch/powerpc/kvm/book3s_interrupts.S>> index f4dd041..2f7d571 100644>> --- a/arch/powerpc/kvm/book3s_interrupts.S>> +++ b/arch/powerpc/kvm/book3s_interrupts.S>> @@ -132,9 +132,18 @@ kvm_start_lightweight:>> *>> */>> >> + PPC_LL r3, GPR4(r1) /* vcpu pointer */>> +>> + /*>> + * kvmppc_copy_from_svcpu can clobber volatile registers, save>> + * r14 to get a spare scratch register for the exit handler id.>> + */>> + PPC_STL r14, VCPU_GPR(R14)(r3)>> + mr r14, r12> > In the case where kvmppc_handle_exit_pr returns RESUME_GUEST and we do> the lightweight guest re-entry, I don't see anywhere that we re-load> r14 with the guest value. So I think this results in guest-visible> corruption of r14.
Ugh. You're right.
> Why not just use the vcpu->arch.trap field? Store r12 to> VCPU_TRAP(r3) here and load r5 from VCPU_TRAP(r4) just before calling> kvmppc_handle_exit_pr().
I was trying to be smart and not do a memory access for a field that I always need. But then again I always do need r14 just as well, so it doesn't actually make a difference. I'll change it.
Thanks a lot!
Alex
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